CA1064581A - Pulse control circuit and method for electrosurgical units - Google Patents

Abstract

Pulse Control Circuit and Method for Electrosurgical Units Abstract:
A control circuit for electrosurgical units establishes a particular output signal to patient electrodes in response to condition of the patient electrodes. The duty cycle of the output signal is reduced when the patient electrodes are not in contact with the patient so as to prevent unwanted cutting and the duty cycle is increased when both patient electrodes are in contact with the patient so as to maximize the coagulation effect.
The invention includes method of adjusting coagulation power by varying the duty cycle and not the amplitude of the output signal.

Description

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Th,is invention relate.s gerleral1.y to r~ no(l.u]cl'c:ion circuits and more particularly concerns rf modu].a-tiorl circuit~
for use in electrosurc3ical units. Electroswra.ical units use ~ high frequency (:RF) power for cutt:ing and coayulation of tisslle : under surgical conditions. The e].ectrosurgical units anply ,.
a high requency alternatinc.~ current at power l.evels up to several hundred watts to electrodes usually consisting of an active pxohe ; and a dispersive plate yenerally known as a patient plate.
Two main types of current are providecl, one for cuttintJ
a.nd one for coayulation. The optimal cuttinq current is a :, ;' continuous wave output from the electrosurg.ical unit. For ~,., smooth cutting a continuous arc is requirecl bt-~tween the active probe and the patient. Upon appli.cation of a high powex continuous wave arc, the tissue cells volati~e resulting in a `` smooth cuttinCJ action as the prohe is movec1 a,long the surface oE
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the tissue. To introcluce hemostasis, the cutti.n~ current ~,he ~ wave form is pulsed. The lower the duky cycle, the greater ~

-,;~i ~ be the amount of hemostasis ant~ -the less the cuttiny effect. Duty ;:

~ cycle is defined as the ratio o:E pulse on time to ct.llration of the :,.
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~ total pulse times 100~. For effective coagulat.ion a current with .. , . ~ .
a duty cycle of approximate:ly 20~ to less than 5~., i9 required. Jj' The longer o~f-time with a low cluty cycle allows the tissue to cool o~f, 90 as to avoid volati~ation o:E cells, but enough power must be applied t,o sear of~ exposed blood vessels.
Both electrodes art3 availahle .in various configurations to be selected by the sur~eon according to -the intende~ 3e. The . ,' acti.ve probe selectetq by the sur~eon can ran~e in si~e ~rom a pair of ~orceps or a knife blade to a ine neet~lt-~. The contact ,, are~ o the probe and the type of tissue encounterecl are factors determinint3 the amount of power necessary to eEfectivel~ cut or ' .
. ., ~ coagulate the blood vessels contigous to the operatin~ situs.
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ELectrosurgical units have previously used either spaxk gap or vacuum tube methods to achieve radio freqllency levels of several hundred watts. For many years the generator used for producing a coagulation current was a spark gap t~pe of generator. A spark gap oscillator can generate ]arge peak powers at a low duty cycle while maintaining about 120 wat-ts of average power. Spark gap methods, however, generate white noise whereas spectrum purity is desirable with electrosu~gical units, particularly since electronic equipment is becoming more prevalent in hospitals. Vacuum tube units are capable of generating a power output of several hundred watts in the megahertz range, but, they generally also operate at low efficiency and have low reliability compared to presently available solid state circuitry. With the advent of solid state units it has been found that presently available transistors cannot generate the large amounts of peak power required under some conditions. Hence, so the duty cycle had to be increased to allow for adequate average power, but, the . ,1 , .
- ~ larger duty cycle introduced a cutting effect in the coagulation mode. To minimize the cutting effect in the coagulation mode, ~ ;
a low duty cycle is required.
, ' The amount of po~er required varies depending upon whether the active probe is axcing or in physical contact with f the tissue and is also dependent upon the effective current density at the operating site, as determined by -the contact area of the probe. All electrosurgical units on the market today employ amplitude control to vary the amount of coagulation power sin~e a low duty cycle results in less cutting effect it .,i . .
~! would~ therefore~ be desirable to vary the duty cycle of electro-surgicaI units in response to load conditions as opposed to :1 varying the amplitude control.

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The present invention is used in combination with an electr~surgical unit having a plurali~y of patient electrodes and RF generating means for providing an output volta~e across the electrodes, and relates to a pulse control circuit connected to the electrosurgical unit for pulse modulating the output signal and controlling the duty cycle of the pulse modulated output signal applied to the plurality of patient electrodes. The pulse control circuit comprises: voltage circuit means for sampling the magnitude voltage between the patient electrodes;
threshold circuit means for generating a threshold signal when the magnitude of the voltage between the patient electrodes e~ceeds a predetermined level, and modulator circuit means ~or pulse modulatlng the output signal at a first duty cycle in the absence of the threshold signal and at a seeond duty cycle in response to the threshold signal.

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In its method aspect, the lnvention relates to a method of controlling, during operating conditions, the ~20 ~ average power of a pulse modulated output signal on an .~ . ..
electrosurgical unlt which comprises the following steps:
selecting the mode of operation; providing a pulse modulated output signal having a constant amplitude; and varylng ,.. . .. .
~ the duty cycle of thè output signal to obtain the amount 5~!
:~ of power desiretl for the particular opera~lng condltions : while allowing the amplitude to remain substantlally constant.
The electrosurgical unit may alæo be manually ~ operated so as to control the average power by providing a :~ pulse modulated oukput signal having a conætant amplitude 30 : ~ and varylng ~he duty cycle o~ the outpu~ signal~
Figure 1 is a block diagram of an electrosurgical unit ~hich includes the pulse control circuit of ~he present invention.

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Fig~lre 2 is a scllematic representation of the pulse control circuit of Fig~re 1.
Figure 1 is a block diagram of an electrosurgical unit which includes the pulse rontrol circuit of the present invention. An oscillator 10 generates a continuous wave RF
signal. The RF signals are applied to the input of an amplifier 12. A modulator circuit 14 which drives amplifier 12 on and off. The result is that the RF slgnal is pulse modulated by the amplifier 12 as driven by the modulator circuit 14. Control means 16 ls used by the operator to select the desired modulation mode sultable for the surgical functions of cutting, cutting with hemostasis 7 and coagulation which are dependent upon the shape of output wave form and the duty cycle~ A power ampl~fier 18 amplifies the modulated signals from amplifier 12 to a power level of appro~imately :
400 watt~. The amp].ifier signals are coupled from the power amplifier 18 by a transformer 20 to a .

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!!r~ mb/J~J - 4a 3?air Oe patient electrot.les whlch inc~urle an acki.ve prnhe 22 an~-l a patient plate 2~. The patien-t ~fj main-tairls continua]. contact with the patient plate 2~ dur;.n~ til~ suryic~1 oper~tion. The active probe 22 is use~ Eor sur~ical procedure~.
In accorclance with a :first feat~lre of the invention, two capacitnrs 28 and 30 are connected in series hc~kween l-.he active probe 22 and the patient p].ate 2aO rrhe purpose of the ca~acitors 28 and 30 is to act as a volta~ ~.i.vitler so as to sal~ple ~he voltage potential between the acti.ve prohe 22 and the pati.ent plate ~. It should be notecl that other volta.cJe dividers such as two resistors or othex elements could also he used in .
this fash.ion insteacl of capacitors. The input terminals o~ a threshold circuit 32 are connected across the capacitor 30.
~;, It i5 seen that the volta~e across the input of the threshold circuit 3~ is the same voltage that appears across capacitor 'i 30.
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~ The property of the thxeshold ci.rcuit 32 is to ~enerate ;, ~ an out~?ut signal when the input signal exceeds a preset ma~nitucle.
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The input voltage will be of low ma~nit:iude or high ma~nitlldt-~
-; depending upon wheth~r the active prohe 22 is or i5 not in contact with the patient 26. When the act.ive prohe 2~ is not in contac~
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with the patient 26 ~he voltage across capacitor 30 will he in the high state and of sufficient magnitude to cause the thresholcl 3' circuit 3Z to therehy generate an output .signal. The output 'I
I si~nal :Erom the k]~resholtl circuit 32 is connected to the moclulated circuits 14 through an enablin~ circuit 34. The enahling circuit 3~ is enabled by the control circuit 160 When the control circuit 16 is set by the operator so as to be in the~ coaglllatin~
mode the ena~le c:ircuit 34 will be turned on so as to allow t3,1e ollt~ut ~rom tlle threshold circult to reach the modulator c.ircuit ~:~
14. In the coagu:Lation mode, modulator circuit l~ can he ad]usted to~modulate the signal, for example, with 20~ du-ty cycle .in . . . . .. . ..... .
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the ahsence of a threshol~1 OUtpllt signal. As ~i]l he descrih~d in further de-tail in a la-ter portion o~ the ~pec;~ication tne pxesence o~ a thresholcl signal has the affect of reducing the cluty cyc7.e of -the RE' slgnal, for example, rom 20,~ to 5~. Thls reduction in duty cycle re~ctins in effect until the acti~e ~robe 22 becomes in con-tact with the patient 26 at which time the voltage across capacitor 30 and input si~nal to the threshold circuit 32 drops preventiny the generation of t'he threshold ;' output signal. The removal oE the threshold outpu-t signal allows the modulated RF signal to return to the higher pret1eter~ined duty c,vcle of 20.; in our example. The purpose of controlling o the duty cycle is to avoid cutting of the patient tissue while the electrosurgical unit is in the coagulation mode. It has been found that substantial cutting will occur when the active prohe is not in contact with the patient but when at such a distance as to substain an arc between the active prohe 22 an~ the patient 26.
During coac,lulation it is desirable to reauce the dut~ cycle under arcing condition by reducing the average power ~issipated at the operating site thereby reducing the cutting efectO When the .; . , , active prvbe has made contact with the patient 26 a high power level is permissible as there is no longer an arc substained so that unwanted cutting is eliminatecl. The higher average power i5 desirable to obtain the desired coayulation. When the control circuit 16 i.s switched to be in the cutting mo-le, the enable circuit 34 is disctblec1 SO as to pxevent the threshol-l OUtptlt signal from the t,hreshold circuit 32 from reaching the moclul~tion cir~uit 14, thereby the duty cycle o~ the RF sic3nal remains constant regaxdless o~ active probe 22 contact w:ith the pa~ient 26. The duty cycle of the moclulatox l~t ma~ he chan~ed hy khe control circult 16 to 100~ durincJ cuttin~ mode to provide maximum average power under all cutting cond;tions. ~ reduction of this duty cycle while in the cutting mofle wi:Ll pxovicle ~`
hemosta~iis in the cutting mode. '~

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Figure ~ is a schemiltic rer3resentation o~ the threshold circuit 32, the enak31e circuit 3~, -the modulation circ-1it 1~;, the input ampli~ier circuit 12 and the control CirCIlit 16 of Fi~ure 1. The threshold circ~lit 32 includes a conventional Schmidt trigger circuit, however it: is to be understood that other well known threshold circuits means may he llsec1.
rectifier and filter circuit 32B converts the "F voltacJe f~om capacitor 30 to a DC level applied to the Schmid-t circuit 32~.
Schmidt circuits provide a signa~ during the time khe inpu~ ;
voltage attains or exceeds a particular magnitude. Thus, when the voltage across capacitor 30 o~ Figure 1. exceecls ~ particular value an output signal will be genera-ted by the Schmidt trigger.
This output signal is connected through the enahle circuit 3~ to ..
the modulatox circuit 14. The enable circuit 3~ allows the thresholcl .signal to reach the modulatox circuit 14 only when the electrosurgical unit is set to be in the coa~ulation modeO The , , enahle circuit includes an isolation amplifier 3~ o~ conventional 1~ design which is controlled by the coagulation switch 4~. An .,; electromechanical relay or similar device may be used as an enabling circuit means.

The modulating circuit 14 includes an unijunction transistor .1 , 36 connected in an oscillator circuit for gener~ting a sawtooth voltbger the frequency o which is depenclent upr3n resistor ~8 and capacitor 40. The sawtooth voltage is appl.ie~ to the base of a high gain transistor 42. Transistor 4~ acts as a switch and is turned on anfl of~ depending upon the base vc31tage.

Control ci.rcuit 16 applies selected bia.s vc31tc-tges to ~he base of transistor ~ of~setting the sawtooth vo.l.tage thereb~
controllirL~ the ~nount of time transistor ~2 is on. The magnittlle o~ bias voltage is selected by a palr of foot switches ~A arld ~6.
Swi~ch 4~ is closed for coagulatlon and switcll ~6 :Eor cutting.
When switch 46 is closed a hias voltage is appliefl to the hase o.f , ` .. .. . . . . .
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trans.istor 42. The clos.infl c)f swi~ch ~4 sllpplies a blas volta~
to the base of transistor ~2 and 50~ The level of the hl.~s voltage from sax 44 is determined by a potentioMeter ~. As a safety measure, the clo~incJ of coagulation switch ~ turns on a transistor 50 which short circuits the bias vol.tage from SWX A~.
Thus, the coagulation mode overides the cukting mode. Furthermore, when the coagulation switch 44 .iS closed the enable circuit 3~ is enabled allow.iny the output signal from the threshold clrcu;t 32 to be conducted through the ena~le circuit 34 tn the base of transistor 42. The presence o~ the threshold signal voltac3e decreases the on time of transistor 4~. In the absence of a ~ threshold signal the on time of transistor ~2 is determine-l hy the adjustable second bias voltafJe as contxolled hy the potenkiometer 48.
~ mplifier 12 o~ Figure l inclucles transistor 12A connected to transistor 42. The amplifier 12 is enable~1 only ~1hen transistor .
4~ is turnecl on. Therefore, it is seen that the P~ ~ignal from '~, oscillator lO is modulated by transis or 4~ and has i~ pulse width of substantially the same duration as the on time of transistor 42. The total pulse repetition time i5 ~etermined by resistor ',~ :: 3~ and capacitor 40 and remains constant. Therefore~ the duty .-1 `tl cycle of the RP pulse is proportional to the pulse ~/ic1-th of '~ the thre~holcl signal.
i The pulse control circuit heretofore describe~l automatic~lly l controls the duty cycle of the RF sic,.lnal .in respon.se to active ! ~ probe contact with t:he patient when the electrosurgical device in .i coagulation mo~.e. The invention prevent.s unwante-1 cuttincJ durin~
coagulatic)n proceclures by reducinfJ the ~uty cycle ancl-~herehy , reducing the averctfJe power when the electrosurc,Jical unit is .se~.
,.~ fo.r coagula~ion and when the active probe i3 not in contact with the p~tient. When the active pro~e is .i.n contaclt with the '~ patient the danger of cutt;ng i~ .ecluce~l an~.~ t.he ccntrol circuit i 6~,15~
autc~ma-tically increa~ei~ the ~-!uty c~cle the~eb~ lnc~e.~sin~ the average now~r to ~imize the co~crll].atlon effect. r)urirl-J cutkiny . t.he ~ulse cont~ol circuit is dlsah~L.e~ settincr the Gontrol circu.it to cutting mode on]y so that a continuous tta~e .iS
suppliec~ to the prohesO
In further accordance wlth the invention, -the potentio~.eter ~8 may be used to manually control the cluty c~c].e ~.7ithOllt - afEectincJ the amplitude o.~ the RF siynal. rrhii~ method enahleei the operator to adJust the electrosurgical unit to provi-.~e the m.inimum power necessary ~or coa~ulation u~cler the operatinCJ conditions - by using the lo~Jei3t poss.ible ~uty cycle. It hae; been ~ound .. that this procedure reduces unwanted cuttincJ when the electrosurgical unit is in the coagulating moc~e~ This method may be used independently or .in combination with the automatic pulse ~:~ control circuit heretofore described~
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Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In combination with an electrosurgical unit having a plurality of patient electrodes and RF generating means for providing an output voltage across said electrodes, a pulse control circuit connected to said electrosurgical unit for pulse modulating said output signal and controlling the duty cycle of said pulse modulated output signal applied to said plurality of patient electrodes, said pulse control circuit comprising:
voltage circuit means for sampling the magnitude voltage between said patient electrodes;
threshold circuit means for generating a threshold signal when the magnitude of said voltage between said patient electrodes exceeds a predetermined level, and modulator circuit means for pulse modulating said output signal at a first duty cycle in the absence of said threshold signal and at a second duty cycle in response to said threshold signal.

2. A pulse control circuit as defined with claim 1 which further includes:
control circuit means for producing a control signal;
and enabling circuit means interposed between said threshold circuit means and said modulating circuit means, said enabling circuit means being in communication with said control circuit means and responsive to said control signal for connecting said threshold signal with said modulating circuit means.

3. A pulse control circuit as defined in claim 1 wherein:
said voltage circuit means includes two capacitors connected in series between said patient electrodes.

4. A pulse control circuit as defined in claim 1 wherein said modulator circuit means includes:
a transistor, and sawtooth generating means for supplying a periodic sawtooth signal to said transistor, said transistor also being coupled with said threshold circuit means whereby the conductive state of said transistor is determined by said sawtooth signal and said threshold signal;
wherein said electrosurgical unit includes RF signal amplifier means connected to said transistor and controlled as said transistor so that when said transistor is conductive said RF signal amplifier means is operative.

5. A pulse control circuit as defined as claim 1 wherein said first duty cycle is higher than said second duty cycle.

6. The method of controlling, during operating conditions, the average power of a pulse modulated output signal on an electrosurgical unit which comprises the following steps:
selecting the mode of operation;
providing a pulse modulated output signal having a constant amplitude; and varying the duty cycle of said output signal to obtain the amount of power desired for the particular operating conditions while allowing the amplitude to remain substantially constant.